Raman Laser Displays 62-µW Threshold

Raman lasers promise applications in telecommunications as sources for component testing and as pumps for erbium-doped fiber amplifiers, but they have tended to require significant input powers. Researchers at California Institute of Technology in Pasadena, however, have constructed a next-generation Raman source based on micro-spheres that displays a pump threshold of 62 µW and a differential quantum efficiency of 36 percent, enabling it to emit radiation using very low pump powers in a footprint 10 times smaller than a laser diode.

Researchers at California Institute of Technology have constructed an ultralow-threshold Raman laser based on silica microcavities and a tapered optical fiber. By coupling the resulting output to other microcavity Raman lasers, the wavelength range of the original pump source may be significantly extended. Courtesy of Kerry J. Vahala.Unlike traditional lasers that employ population inversion, Raman sources produce laser radiation as a result of molecular distortion in the active region caused by optical stimulation, explained Kerry J. Vahala, a professor at the university. If the pump intensities are sufficiently great, a new optical wave forms at a different wavelength, enabling Raman sources to be used to extend the wavelength range of other lasers. By using the generated Raman waves as the pump for other Raman sources in a process called cascading, the wavelength range may be extended even farther.

To decrease the threshold, Vahala and graduate students Tobias Kippenberg and Sean Spillane employed silica microspheres 28 to 110 µm in diameter as an optical resonator, which they produced by melting commercial fiber with a CO2 laser. Despite the fact that silica has a much smaller Raman gain coefficient than other materials in similar studies, such as CS2 microdroplets, its very high Q-factor ensured that even a low input power remained trapped long enough in the cavities to build to the necessarily high intensities. In the tests, the Q-factor of the microspheres was greater than 108.

"Our microsphere Raman laser uses the physics of spheres to first wrap a large effective interaction length into a small ball and then to greatly magnify power concentrations; i.e., tight wrapping of the orbits," Vahala said. "The result is much higher efficiency in a much smaller device."

Crucially, the researchers directly coupled the pump input from the 1550-nm external-cavity laser diode pump to the whispering gallery mode of the spheres using a fiber optic taper. They controlled the position of the fiber with a three-axis stage with a 20-nm resolution. An undercoupled air gap of 0.15 µm yielded the minimum threshold.

Spheres are ahead

Substantial work remains before microcavity-based Raman lasers could find commercial expression, Vahala said. Using spheres for this application results in thermal drift, and their replacement with microdiscs would be ideal.

"We would like to apply the idea in a disc geometry to have a more controllable mode spectrum, but as of yet, discs do not meet the stringent Q requirements -- spheres are orders of magnitude ahead," he said.